1. Field of the Invention
This invention relates to a pneumatic circuit for a wheel mounted shovel which includes a working/service brake device, a parking brake device and a suspension locking device which are controlled pneumatically.
2. Description of the Prior Art
Various pneumatic circuits for a wheel mounted shovel of the type mentioned are conventionally known. An exemplary one of such conventional pneumatic circuits is disclosed, for example, in Japanese Utility Model Publication No. 63-5978. The exemplary pneumatic circuit is shown in FIG. 3. Referring to FIG. 3, the pneumatic circuit shown includes a pair of working/service brake devices 1, a parking brake device 2 and a suspension locking device 3 which are all controlled pneumatically. When the wheel mounted shovel is to work, the working/service brake device 1, parking brake device 2 and suspension locking device 3 are all rendered operative to carry out individual braking and locking operations. But during travelling of the wheel mounted shovel, the devices 1, 2 and 3 described above do not carry out their braking or locking operation, and upon parking of the vehicle, the working/service brake device 1 does not carry out a braking operation while the parking brake device 2 and the suspension locking device 3 carry out a braking operation and a locking operation, respectively.
The pneumatic circuit shown in FIG. 3 further includes a foot pedal 4, a service brake valve 5, a working brake valve 6, a solenoid operated parking brake valve 7, a solenoid operated suspension control valve 8, a suspension locking valve 9, a pair of suspension locking cylinders 10, a booster 11 for the working/service brake device 1, a shuttle valve 12, a compression pump 13 serving as a pressure source, and an air tank 14. The pneumatic circuit is connected to an electric circuit for controlling the valves 6, 7 and 8. The electric circuit includes a main switch 15, a change-over switch 16, an on/off switch 17, and a power source 18.
As diagrammatically shown in FIG. 3, the service brake valve 5 and the working brake valve 6 are pneumatically connected to the working/service brake devices 1 by way of an air passage, and the parking brake valve 7 is pneumatically connected to the parking brake device 2 by way of another air passage while the suspension control valve 8 is connected to the suspension locking valve 9 by way of a further air passage. The three air passages described just above extend through a swivel joint (not shown) provided in a swing coupling section (not shown) of the wheel mounted shovel. Since the air passages individually have considerably great sectional areas, also the swivel joint has a great overall size. Further, since the parking brake valve 7 and the working brake valve 6 are provided separately from each other in order to allow the parking brake device 2 to function independently, a correspondingly high cost is required for the pneumatic circuit and a considerably great spacing is required for the valves 7 and 6. Besides, the on/off switch 17 for the suspension control valve 8 is normally held in an energized condition but is deenergized into an open condition only when the suspension locking device 3 is to be held in a locking condition during travelling of the wheel mounted shovel. Such normally energized condition is not preferable for the duration of the on/off switch 17.